Set limit on account

[#1]

I want to set up a limit of 15 Pounds on my account, which means, I cannot spent more than 15 Pounds on calls. How can I do that?

[#2]

You can't reduce it from the portal
You would need to raise a ticket
HOWEVER
If you reach the limit your line will be blocked and you will get hit with a charge of £5.76 http://www.plus.net/support/phone/credit-limit-and-top-ups-guide.shtml

[#3]

Also because of the delay in Plusnet processing the calls, you could make loads more calls after the one that puts you over the limit and Plusnet implementing the block.